For the transfer of patterns of integrated circuits such as ICs and LSIs, a reduction projection aligner or a photolithography apparatus is mainly used. With the enhancement of integration degrees of integrated circuits, a wide exposure area and a high resolution over the entire exposure area are required for a projection optical system used in such an apparatus. In order to further improve the resolution of the projection optical system, it can be considered to further reduce the exposure wavelength or increase the numerical aperture (NA) of the projection optical system. At present, KrF (248 nm) excimer lasers are mainly used as light sources of exposure apparatuses. In order to further improve the resolution, KrF excimer lasers, which are deep ultraviolet rays having shorter wavelength, have been developed.
With knowledge in known excimer laser lithography technology, damages of optical components are mostly classified broadly into three types. The first type is a reduction in internal transmittance caused by the generation of internal defects (see Japanese Patent Application Laid-Open Gazette No. Hei 8-259255). The second type is a refractive index change associated with a volume change such as compaction or expansion (see International Application National-Phase Publication No. 2000-517284). The third type is a surface state change such as the destruction or contamination of an optical thin film (see Japanese Patent Application Laid-Open Gazette Nos. 2002-14202 and Hei 11-52102).
However, damage which does not belong to any of them has been recently reported and attracted interest (see E. M. Wright et al., “Spatial pattern microchannel formation in silica glass irradiated by nanosecond ultraviolet pulses”, Applied Optics, 1999, Vol. 38, p. 5785–5788). This is the microchannel phenomenon in which holes having diameters of several μm are formed from a surface of an optical component to the inside thereof. It is expected that the occurrence of this phenomenon causes a reduction in the transmittance of the optical component resulting from scattering and the destruction of an optical thin film, and significantly reduces transmittance characteristics of the optical component.
However, there are few reports on the microchannel phenomenon, and sufficient knowledge has not been obtained yet. It has been known that this microchannel phenomenon has the following characteristics:
(1) It occurs when 109 pulses of an ArF excimer laser having an energy density of several mJ·cm−2·pulse−1 are continued to be applied to synthetic silica glass.
(2) It occurs not at a face from which the laser is incident but at a back face.
(3) It occurs in the vicinity of the boundary between a portion irradiated with the laser and a non-irradiated portion.
This microchannel phenomenon occurs due to extremely long continuous irradiation homogeneous at low energy densities in a practical range under normal use conditions of an exposure apparatus. Accordingly, it is necessary to take prevention measures against mechanical damage resulting from this phenomenon.